define and implement adjoint operator
allows to trivially  the adjoint operator and transpose the matrix to obtain it
update issue #198: removed some FEELPP_ENABLE_MPI_MODE
use adjoint, update issue #161
updare issue #161: seems to work. more check needed.
 - implementation of sibling in stencil 

@vincentchabannes siblings are meshes extracted from the same mesh.
I had to do that because something is not right with work with parallel
mesh in sequential spaces